Can South Korea Build a Nuclear Bomb in 6 Months?

As tensions with North Korea are heating up, a prominent South Korean nuclear physicist said his country can build a nuclear weapon in just six months.

In a newly released Korean-language article in Chosun Monthly, Kune Y. Suh, an MIT-trained nuclear engineer at Seoul National University, argues that South Korea needs to build its own nuclear arsenal to deal with the threat posed by Pyongyang. When asked if Seoul is capable of producing such a weapon, Suh argued that the country could use its huge stockpile of plutonium to build an operational nuclear weapon in six months.

Suh didn’t go into detail about how Seoul could accomplish this feat, which is notable because its plutonium stockpile largely consists of what many dismiss as “unusable” reactor-grade plutonium. But clues to how South Korea could overcome this challenge were offered at an event our employer, the Nonproliferation Policy Education Center, hosted on Capitol Hill on September 14 for a visiting delegation of Japanese Diet members. That event, which focused on the plutonium stockpiles in northeast Asia, featured an eye-opening presentation by Bruce Goodwin, a Senior Fellow at the Center for Global Security Research at the Lawrence Livermore National Laboratory. At the event, Goodwin, one of America’s top nuclear weapons designers and plutonium experts, thoroughly debunked the myth that countries cannot use so-called reactor-grade plutonium to build nuclear weapons.

Reactor-grade plutonium refers to plutonium that has a larger percentage of the isotope Pu-240 compared with weapons-grade plutonium, which has a higher mix of the isotope, Pu-239. Many have contended that the high level of Pu-240 makes reactor-grade plutonium unsuitable for nuclear weapons because it would require too large of a critical mass, is too radioactive to handle, and is prone to a condition called “pre-initiation,” which is when the nuclear chain reaction begins too soon producing a minimal explosive yield. As the World Nuclear Association has put it, “any significant proportions of Pu-240 in it would make it hazardous to the bomb makers, as well as probably unreliable and unpredictable” for use in nuclear weapons.

Goodwin’s presentation exploded each myth, which persist even though the United States tested a reactor-grade-plutonium nuclear device in 1962. First, he noted that the critical mass for reactor-grade plutonium in nuclear weapons is only two kilograms larger than weapons-grade plutonium, and much less than using highly enriched uranium. The reason why many believe a much larger critical mass is needed with reactor-grade plutonium is because Pu-240 is not very fissionable with the slow neutrons present in nuclear reactors. On the other hand, Goodwin explained, “Pu-240 is a good fissile material with fast neutrons and fast neutrons drive nuclear explosives.” The irony here, Goodwin pointed out, is that reactor-grade plutonium works better for nuclear weapons than in reactors.

The radioactivity differences between reactor and weapons-grade plutonium are also negligible. To be sure, reactor-grade plutonium is over three times as radioactive as its weapons grade counterpart, but the differences would have to be “several factors of ten” to seriously inhibit handling of the material. In fact, Goodwin’s presentation noted, “In newly built facilities, handling reactor grade is essentially the same as handling weapons grade Pu.”

The problem of pre-initiation, while real, is also overblown. As Goodwin points out, the Department of Energy released a report in 1997 that stated that “even if pre-initiation occurs at the worst possible moment (when the material first becomes compressed enough to sustain a chain reaction) the explosive yield of even a relatively simple first-generation nuclear device would be of the order of one or a few kilotons.” This is the absolute minimum yield, and in all likelihood, it would be much higher even for a first-generation device. Nonetheless, “a one-kiloton bomb would still have a radius of destruction roughly one-third that of the Hiroshima weapon, making it a potentially fearsome explosive.”